webnn: use strongalias for OperandId

Use StrongAlias for OperandId and typemap it to struct in mojo. Still
use the raw uint32 for GraphInfo.constant_operand_ids_to_handles map
because StrongAlias doesn't implement WTF::HashTraits, so can't be used
as keys in WTF::HashMap.

Change-Id: I1545b908a4030fe3f33c76fd0c0165f1f6563bca
Bug: 413722115
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6514828
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Phillis Tang <phillis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1462333}